The present invention relates to a process for producing silicon tetrafluoride of high purity, which is used in the fields of electronics, optics and the like.
Silicon tetrafluoride (SiF4) is used, for example, as a fluorine-doping agent of quartz-based optical fibers, as a raw material of photomask materials for semiconductor lithography, and as a chemical vapor deposition gas for producing semiconductors. Silicon tetrafluoride has increasingly been used in recent years. Therefore, there is a demand for more efficient techniques for producing silicon tetrafluoride of high purity.
There are several conventional processes for producing silicon tetrafluoride, as follows. Japanese Patent Unexamined Publication JP-A-63-74910 discloses a first process for producing silicon tetrafluoride by pyrolyzing a metal silicofluoride (e.g., sodium silicofluoride). Japanese Patent Unexamined Publication JP-A-57-17414 discloses a second process for producing silicon tetrafluoride by introducing hydrogen fluoride gas into a system where amorphous silicon dioxide has been dispersed in sulfuric acid. Japanese Patent Unexamined Publication JP-A-7-81903 discloses a third process for producing silicon tetrafluoride by reacting fluorine with a mixture of calcium fluoride and silicon powder at 140xc2x0 C. Of these processes, the third process is superior to the first and second processes with respect to reducing the amount of industrial wastes. In fact, the third process does almost not generate wastes. In contrast, the first and second processes respectively generate large amounts of a metal fluoride (e.g. sodium fluoride) and sulfuric acid as industrial wastes. Prior to conducting the third process, it is necessary to produce fluorine by electrolyzing hydrogen fluoride. Due to this, the third process is disadvantageous in terms of reducing production cost and energy consumption.
It is an object of the present invention to provide a process for economically producing silicon tetrafluoride of high purity.
According to the present invention, there is provided a process for producing silicon tetrafluoride. This process comprises the step of (a) reacting at a temperature of 250xc2x0 C. or higher elemental silicon in the form of solid with hydrogen fluoride in the form of gas, thereby producing a gas product comprising the silicon tetrafluoride
In course of achieving the present invention, the present inventors have studied the use of hydrogen fluoride, which is superior to fluorine, as the fluorine source in producing silicon tetrafluoride from silicon. In fact, hydrogen fluoride is superior to fluorine by the following points. Firstly, hydrogen fluoride itself is a raw material for producing fluorine by electrolysis. Therefore, the use of hydrogen fluoride is naturally lower than that of fluorine in cost. Secondly, hydrogen fluoride is lower than fluorine in chemical potential. Therefore, a reaction of hydrogen fluoride with silicon is considered to generate a less heat than that of a reaction of fluorine with silicon. Thus, it is easier to conduct a thermal control of the former reaction as compared with the latter reaction. Thirdly, hydrogen fluoride has a boiling point of 20xc2x0 C., thereby making it possible to store hydrogen fluoride in the form of liquid under normal temperature below its boiling point. In contrast, fluorine is in the form of gas under normal temperature. Therefore, the handling of hydrogen fluoride is much easier than that of fluorine.
In reacting silicon with hydrogen fluoride, it can be understood in theory that any of the following three reactions (1)-(3) can occur since xcex94G of each reaction has a negative value.
Si (solid)+4HF (gas)xe2x86x92SiF4 (gas)+2H2 (gas)xe2x80x83xe2x80x83(1)
Si (solid)+3HF (gas)xe2x86x92SiHF3 (gas)+H2 (gas)xe2x80x83xe2x80x83(2)
Si (solid)+2HF (gas)xe2x86x92SiH2F2 (gas)xe2x80x83xe2x80x83(3)
The present inventors, however, unexpectedly found that, when silicon in the form of solid is actually reacted with gaseous hydrogen fluoride, the above reaction (1) occurs dominantly, thereby efficiently producing silicon tetrafluoride.
Although the above reaction (1) does almost not occur at a temperature around room temperature, it does occur as the temperature increases. In fact, the reaction rate increases rapidly as it passes about 250xc2x0 C. The reaction rate further increases as the temperature increases further. The inventors found that the above reaction (1) occurs dominantly in a temperature range of from 300xc2x0 C. to a temperature higher than 1,000xc2x0 C. by directly reacting silicon with hydrogen fluoride gas.
By a further detailed examination on the reaction between silicon and hydrogen fluoride, the inventors found that the above reaction (2) also occurs in some cases, although the reaction (1) is a dominant reaction. In these cases, the gas product naturally contains SiHF3. As the reaction (1) proceeds, H2 is formed as a by-product in an amount two times that of SiF4 in molar number, thereby making the reaction system under a strongly reduced atmosphere. The inventors assumed that SiHF3 would be formed when the amount of HF is insufficient under the H2 atmosphere. Thus, the inventors have tried to suppress the formation of SiHF3 by leaving a part of HF (the unreacted HF) in the resulting gas product. With this, the inventors unexpectedly found that SiHF3 is not formed when the gas product contains HF in an amount of at least 0.02 volume %, preferably at least 0.05 volume %. In other words, according to a first process of a first aspect of the present invention, the reaction is conducted in a manner that the gas product contains at least 0.02 volume % of the unreacted HF, in order to produce silicon tetrafluoride of high purity. Although the upper limit of the HF content of the gas product is not particularly limited, it is preferably 1 volume % or lower in order to conduct a post-treatment for removing HF in an economical way.
In order to control the HF concentration of the gas product in accordance with the first aspect of the present invention, it is preferable to use a so-called perfect mixing type reactor. With this, it becomes possible to obtain a uniform composition of the gas phase through a compulsory stirring of the inside of the reactor. In fact, a large amount of HF gas always exists around Si particles when the gas phase of the reactor is under a perfectly mixed condition. With this, the gas phase HF concentration is less influenced, even if the flow rate of HF gas supplied to the reactor or the reaction temperature fluctuates by certain degrees.
When the gas product contains HF in an amount of 0.02 volume % or higher, such HF can be removed from the gas product by a post-treatment. This removal can be conducted by a purification of the gas product. It is possible to conduct such post-treatment, for example, by bringing the gas product (containing HF) into contact with NaF (in the form of pellets), thereby fixing HF in the form of NaF.HF. It is possible to conduct the post-treatment in an economical way, if the HF concentration of the gas product is in a range of 0.02-1 volume % or 0.05-1 volume %.
When the gas product contains no HF or a very small amount of HF, the gas product may contain SiHF3 as a by-product. The SiHF3 concentration of the gas product becomes higher as the HF concentration of the gas product becomes lower. For example, when the gas product contains no HF, the SiHF3 concentration may reach about 1 volume %. Such condition can be obtained, for example, by conducting a reaction in a manner that HF gas is introduced in a so-called piston flow manner into one end of a cylindrical reactor charged with silicon. In fact, the HF gas introduced into the reactor is consumed by the reaction with silicon as it passes through a fixed bed of Si. Therefore, the HF concentration of the gas phase becomes zero as the HF gas introduced in a piston flow manner reaches the exit of the reactor, provided that silicon exists in a sufficient amount relative to that of the introduced HF gas.
The above-mentioned SiHF3, which may be contained as a by-product in the gas product, is thermally unstable as compared with silicon tetrafluoride. Therefore, it is possible to conduct a disproportionation of SiHF3 into Si and SiF4 by heating the gas product obtained at the exit of the reactor, as shown by the following reaction formula (4).
4SiHF3xe2x86x92Si+3SiF4+2H2xe2x80x83xe2x80x83(4)
The inventors found that this disproportionation does not proceed well only by heating the gas product. As a result of an elaborate examination on the conditions for allowing the reaction (4) to proceed in the right direction, the inventors unexpectedly found that it is effective to treat the gas product (containing SiHF3) with a heated elemental (metallic) nickel. In fact, we unexpectedly found that the above disproportionation occurs easily and selectively by bringing the gas product (containing SiHF3) into contact with elemental nickel at a temperature of 600xc2x0 C. or higher. This is a second process according to a second aspect of the present invention. After its use for this disproportionation, the surface of nickel was found to have a nickel suicide represented by Ni31Si12. In case that nickel exists in the reaction system, silicon formed by the disproportionation reacts with Ni. The resulting Ni31Si12 is separated from the reaction system (gas phase), thereby losing the chemical equilibrium of the reaction in a manner to allow the reaction to proceed in the right direction. The nickel filler used in the disproportionation may take an arbitrary shape. It is, however, preferable to take measures to make the nickel have a large surface area capable of its sufficient contact with the gas product.
As stated above, the above disproportionation is conducted, when the gas product contains no HF or a very small amount of HF, in order to remove SiHF3 from the gas product. Therefore, the gas product after completing the disproportionation is substantially free from impurities (e.g., HF and SiHF3). Thus, it is not necessary to conduct a purification for removing HF (e.g., by using NaF), since the gas product is already substantially free of HF.
As an alternative to the above disproportionation, the inventors unexpectedly found that, when the gas product (obtained by the reaction between Si and HF) contains SiHF3 as a by-product and is substantially free of hydrogen fluoride, it is possible to remove SiHF3 by a third process according to a third aspect of the present invention. The third process comprises the steps of:
adding at least 0.1 volume % of hydrogen fluoride to the gas product, thereby preparing a gas mixture containing SiHF3 and the hydrogen fluoride; and
bringing the gas mixture into contact with elemental nickel at a temperature of 400xc2x0 C. or higher, thereby reacting the SiHF3 with the hydrogen fluoride to produce silicon tetrafluoride, as shown by the following reaction formula (5). 
As mentioned above, it is possible to obtain the gas product, which contains SiHF3 as a by-product and is substantially free of HF, for example, by introducing HF into one end of the cylindrical reactor (charged with silicon) by a piston flow manner. In the third process, it is preferable to add hydrogen fluoride in an amount of 0.1-1 volume %.
In the third process, it is important to appropriately control the HF concentration of the reaction system in order to completely turn SiHF3 into SiF4. It is, however, easy to adjust the HF concentration in the third process. In fact, it is more easily possible in the third process to maintain the HF concentration at a constant level, since HF with a constant flow rate is added to the gas product substantially free of HF, as compared with the first process in which the HF concentration is indirectly adjusted by the amount of the unreacted HF remaining in the gas product. In the third process, the nickel filler does not turn into a nickel silicide, but is maintained in its metallic or elemental form. Furthermore, the temperature for conducting the reaction (5) can be substantially lower than that for conducting the reaction (4). In order to completely turn SiHF3 into SiF4 in the third process, it is necessary to add HF in an amount equimolar with that of SiHF3 or greater. Similar to the first process, it is necessary to remove HF, if it remains in the system after the reaction (5).
By eagerly examining the third process, the inventors unexpectedly found that it is possible to remove other four impurities (i.e., SiF3CH3, C2H6, C2H4 and (SiF3)2O) together with SiHF3 by conducting the third process. Of these four impurities, the carbon-containing components (i.e., SiF3CH3, C2H6 and C2H4) originate from carbon contained as an impurity in silicon as the raw material. It may be difficult to separate these carbon-containing components from SiF4 by other conventional measures except the third process. For example, it is difficult to separate the carbon-containing components from SiF4 by using vapor pressure difference therebetween, since the carbon-containing components (i.e., SiF3CH3, C2H6 and C2H4) respectively have boiling points of xe2x88x9230.2xc2x0 C., xe2x88x9288.63xc2x0 C. and xe2x88x92103.71xc2x0 C., which are too high relative to that (xe2x88x9295.7xc2x0 C.) of SiF4. Furthermore, it is almost impossible to remove the carbon-containing components by using an adsorbent such as zeolite and activated carbon. In producing silicon tetrafluoride, it is inevitable to have the above-mentioned carbon-containing impurities in an elemental silicon of relatively low price. As mentioned above, the carbon-containing components are unexpectedly removed in the presence of elemental nickel by conducting the third process, as shown by the following reaction formulas (6), (7) and (8).
SiF3CH3+HFxe2x86x92SiF4+CH4xe2x80x83xe2x80x83(6)
C2H6+H2xe2x86x922CH4xe2x80x83xe2x80x83(7)
C2H4+2H2xe2x86x922CH4xe2x80x83xe2x80x83(8)
It is understood from the reaction formulas (6)-(8) that the final products of these reactions are SiF4 and CH4. Although the reactions (7) and (8) do not require HF, the existence of HF does not interfere with these reactions. It is easily possible to separate CH4 (boiling point: xe2x88x92191.5xc2x0 C.) together with other low-boiling-point components (e.g., H2, O2 and N2) from SiH4 (boiling point: xe2x88x9295.7xc2x0 C.) by vacuum evacuation, since CH4 has a high vapor pressure sufficient for being distributed to the gas phase even at a temperature at which SiF4 liquefies or solidifies.
The above-mentioned fourth impurity (SiF3)2O is generated by water or oxides adsorbed to the raw materials and the reaction vessel and is difficult to be removed. However, this impurity can also be removed by conducting the third process, as shown by the following reaction formula (9).
(SiF3)2O+2HFxe2x86x922SiF4+H2Oxe2x80x83xe2x80x83(9)
It is possible to remove H2O, which is generated by the reaction (9), by using a dehydrating agent such as concentrated sulfuric acid or zeolite.
Thus, as shown by the reaction formulas (5)-(9), the impurities (i.e., SiHF3, SiF3CH3, C2H6, C2H4, and (SiF3)2O), which may be contained in the gas product obtained by the reaction between silicon and hydrogen fluoride, can turn into SiF4, CH4 and H2O by heating at a temperature of 400xc2x0 C. or higher in the presence of HF, H2 and Ni. Then, it is possible to obtain SiF4 of high purity by separating CH4, H2O and H2.